Process for producing fertilizers



Feb. 16, 1937. TROMEL 2,070,697

PROCESS FOR PRODUCING FERTILIZERS Filed Feb. 19, 1936 .B "gum" fillorney:

Patented Feb. 16, 1937 2,070,697 raocass roa rnonccmo ran'rlmzans vGerhard Triimel, Dusseldorf, Germany, assignor to Kaiser-Wilhelm-Institute fur Eiseni'orschung (eingetragcner Verein), Dusseldorf,

Germany Application February 19 In Germany 3 Claims.

Endeavours have long been'made to convert, by calcinatlomthe phosphoric acid contained in natural raw phosphatesoccurrlng. as apatite, I

into a condition assimilable by plants, numerous processes having been suggested for attaining that object. A common feature of all these propmals is the use of high temperature to which the raw phosphates, in association with various added substances, and with or without the presence of steam, are subjected.

With regard to the kind oi such additions, these may be divided into two groups. one, alkali compounds, such as carbonates, sulphates or chlorides, in addition to silicic acid, or also lime, are used. To some extent alkalibearing rocks or minerals, such as phonolite, leucite or the like are also used. In the processes concerned the formation of an alkali-lime phosphate characterizes the course oi thereaction, which therefore implies decomposition by means or" alkalis. By such a process, the socalled Rhenania phosphate .is obtained, this being produced by calcining raw phosphate withallialis at temperatures up to about 1150 C.

In the case oi the second group, on the other hand, the endeavour is to avoid the large additions of alkali compounds, which make the prod not more costly, and to decompose the raw phosphates with the aid oi. silicic acld-in the form of sand or the like-or solely with the aid of alkaline earths, such as lime or the like. To some extent lime and silicic acid are used together, it havingbeen proposed to ruse, along with the raw phosphate, sufilcient lime and silicic acid to-ensure the formation of the compound 5CaOP2'OaSlO2. This process however has not met with success. It is true that this compound dissolves readily in; citric acid or solutions of ammonium citrate; nevertheless there is no noteworthy amount of the compound formed from natural raw phosphates under manufacturing conditions.

In decomposing raw phosphates by calcinationwithout -alkali it is important that fluorine should be largely eliminated by the formation of volatile compounds; so longas it forms compounds of the are practically insoluble. to made to expel the fluorine have -1'ailed in prac-" tice chiefly-for two reasons. In

fluorine is present expelling the fluorine in a volatile form, a state 1936, Serial No. March 26, 1935 4 dustrial and Engineering Chemistry, vol. 27

. composed product was obtained.

the first p ace, my'more recent researches have'shown that, in

event no lumps or incru'station is .iormed in the and the gaseous phase, so that, even at temperatures of 1400-1500 (2., the expulsion is by no means complete. Moreover the expulsion of the fluorine, in operating on a commercial scale, is impeded by the circumstance that the transition 5 of fluorine from the solid to the gaseous phase practically ceases when the phosphate becomes strongly sintered. Sintering chiefly occurs when the phosphate is calcined at temperatures above 1300 to 1350 C., because the fluorine acts as a flux, so that incrustations, which greatly obstruct working, or finally arrest it completely, are formed in the furnace. Moreover the resulting clinker is so hard that it cannot be reduced except by the application of very considerable power.

These two reasons form the chief explanation why the decomposition of raw phosphate-for example in'shait furnaces or rotary-tube furnaceshas hitherto met with no success on a commercial scale, although the decomposition on a laboratory scale has been declared practicable by various publications. This agrees with the statements in a recently published article in In (1935), January issue, page 89, according to which the decompositioncan be efiected in the use oi charges of about 2.5 grms, whereas, in 1 the case of quantities of about 10 grams, no dev Scientific researches with the ternary system, lime-phosphoric acid-silicic acid, have already revealed the existence of another compound, formed of these three components and also displaying very good solubility in citric acid and citrate. This is the compound QCuOPaOeBSiO-a. It has been-found that on adding lime and silicic acid to raw phosphate, in such quantities as approximately to allow the formation of the compound referred to,the phosphoric acid in the raw phosphates readily decomposes, on a com= merclal scale,- by simple calcination. This is, quite unexpected by anyone skilled in the art, especially because-as already mentionedthe're is no appreciable decomposition when raw phosquantities as will produce the compound 5CaO.I='zO5.S iOz, and the mixture is then calapatitetype, whichfvf i The attempts ,hitherclned on a commercial scale. The reactiommixturemade up in accordance 5 with the invention behaves on calcination-Ior ,example in the rotary-tube furnace ina "man nor-entirely difl'erent' from the previously pro. posed mixtures. No sintering, occurs, and in any walls at certain points. In any case the fact is that, on issuing from the furnace, the reaction mixtures according to the invention made from fluorine-bearing raw phosphates, lime and silicic acid, furnish a product which is practically unsintered, but consists of fine-grained, readily friable material, which is almost completely soluble in citric acid. This fact is evidently connected with the circumstance that the fluorine content of the product is below 0.1%, that is to say, an'amoun't that is barely detectable.

The reasons for the behaviour of the mixture according to the invention are not yet quite clear. Of considerable importance A is apparently the fact that the compound consisting of lime, silicic a high speed of and below the fusing point. This reaction is a considerable in-- beunderstood, with the acid and phosphoric acid, formed in accordance with the invention, has a very high fusing point, above 1750 C. Moreover the compound according to the invention seems to be distinguished by formation in the solid mixture manifestly accompanied by crease in volume, which explains the change of the mass into the loose, pulveruient condition. Moreover the large quantities of in the ratio according to the invention appears to facilitate the expulsion of the fluorine in the volatile state. In any event, however this may be, the specified mixture ratio enables the decomposition of fluorine-bearing raw phosphate, by calcination, to be effected, for the first time, in the usual technical furnaces-on a commercial scale, and a product to be obtained that is up to 100% soluble in citric acid; The temperatures to. be employed according to the invention may rise to about 1450 C. Although the use of high temperatures would be possible it.'is by no means necessary, because a completely soluble product .is already obtained. The length of the reaction period varies, as will dimensionsv of the furnace, the amount of the charge, and the temperature employed. In experimental working on a small scale a perfectly satisfactory result can be obtained in 1-2 hours, but in the" case of very large furnaces possibly periods of 5-6 hours or more can be used. In contrast to other processes,

the manner of cooling has no substantial iniiuence on the solubility of the product, and

therefore the cooling may be according to requirements.v

The compound according to the invention, that is, 9CaO.P:Oa.3Si0a, has a certain range ofhomogeneityin theternary system, that is to say,it can take up one or more of its components while at the same time forming mixed crystals. The limits within which quick or gradual,

lie, in the case of CaO. between 58 and 65%; for P205, between about 15-25%, and for SiOz, at, 15-25%.

as is necessary for the hatched area A-,-B-'-C-DE-F represents the ground within which the compound according to the invention occurs,in the pure state or with I in the pure ternaryformation of mixed crystals, system. The limits coincide with those-specified silicic acid used practically no cium compound forming the aforesaid compound-4ra compound with the formula 16CaO.2PzOs.5SiO2 which has the same properties-is exclusively or I predominantly formed The drawing clearlyrevealstiit qiiie enoe tween the compound claimed in the application and the compound 5CaO.P2Qt.S10a. 0n the binary side, CaO-PaOs, two points are given, corresponding to the compounds 4CaO.P2Os and 3CaO.PzOt In raw phosphates, to phosphoric acid lies, as a rule, between these two compounds. Hence if--as has'often been proposed-silicic acid be added to raw phosphates, the composition will be modified in approximate accordance with the broken line leading to the S10: angle of the diagram. Compounds diflering widely from those intended according to the, invention are thus obtained.

It will be understood that these limit values refer to pure substance, leaving impurities out of consideration. In carrying out the process according to the present invention the composition of the original mixture depends on the composition of the raw materials, it being necessary to take into account the losses of H20,CO'2' and S11; during calcination. -The most suitable means for characterizing the compound in question is its Rontgen diagram, since it is distinguished by. adefinite crystalline structure.

The following is an example of the carrying out of the process according to the invention: A Florida pebble phosphate containing 30.7% of P205 was ground with 866 parts of limestone and 235 parts of silversand to a degree of fineness of flour of raw cement. This mixture was heated in an oil heated rotary furnace of the length of 7.56 m. and an internal diameter of 0.7 m., untila temperature of 1450" C. is'reached. A test made on the product showed a content of 17.7% of P205. In a 2% citric acid 17.6% of P205 was soluble, while in an ammoniacal solution of ammonium citrate according to Peter- Although in carrying out the Rhenania process described above an alkali-lime compound is obtained exhibiting the same crystalline structure,

there nevertheless exists between them a decided.

difference inasmuch as, with-theRhenania process. a compound is formed containing approximately 1 mole of NazO or K20, per 1 mole of P205, whereas the present compound requires alkali for its formation.

The requisite lime is advantageously added in the form of calcium carbonate, limestone or the like,.and the *silicic acid as sand 'or the like. However, calcium oxide itself,

may be used. Moreover, calcium oxide and silicic acid already combined as calcium silicate, or slags chiefly consisting thereof, may, alsobe used. It is highly phates such as are, in the substances (lime and silicic acid) to be added.

10%, of alumina,

ent in the original materials, have no influence on the course of the reaction'or the quality of the end product. it is well known in this art,.' alumina and; ferric oxide are adapted to replace v silicic acid, whilst magnesium oxide. can be used} up to 10%, in place'of lime. Theialumina'con tents in the original materials can be rendered the ratio of lime or any other caloxide on calcination,

advisable to use raw 'phosthemselves, already rich in In the process according to the invention low contents, that is to say up. to

useful, in this manner, for the course of the reaction according to the invention; for example, when blast-furnace slag is used as an addition, in the process according to the invention;

In the present process the original substances are advantageously well mixed together prior to their introduction into the furnace. This can be done, for example, by the wet or dry method by grinding together.

In carrying out the procesait is also advisable to operate in an atmosphere of steam, in known manner.

I claim:

1. A process for producing fertilizers on a commercial scale, by calcining raw phosphate with lime and silicic acid, at temperatures up to about 1450 0., characterized by adjusting the relative proportions of raw phosphate, lime and silicic acid in the mixture so that the calcined product contains about 58-65% of CaO, about 15-25% 01 P205 and about 15-25% or 810:.

2. A process for producing-fertilizers on a commercial scale, by calcining raw phosphate with lime and silicic acid, at temperatures up to 1450 (2., characterized by adjusting the relative proportions-o! raw phosphate, lime and siliclc'acid in the mixture, so that compounds with \the formula l6Ca0.2PzOs.5Si0a are obtained.

3. A process for mercial scale, by calcining raw phosphate with lime and silicic acid'at temperatures up to 1450 6.. characterized by adjusting the relative proportions of raw phosphate, lime and siiicic acid in the mixture, so that compounds with the formula 9CaO.P:Ot.3SiO: are obtained.

Gamma]; mom.

producing fertilizers on a oom- 

